1. Field of the Invention
This invention relates to anodized aluminum substrates for electronic package and printed circuit board applications. More particularly, the electrically isolating integrity of an anodic film is enhanced by forming the film from an aluminum alloy having less than threshold amounts of precipitate forming elements including iron, silicon and manganese.
2. Description of the Related Art
Anodized aluminum substrates are used as components in packages that encase electronic devices as disclosed in U.S. Pat. No. 4,939,316 to Mahulikar et al, that is incorporated by reference in its entirety herein. The U.S. Pat. No. 4,939,316 patent discloses an adhesively sealed electronic package having anodized aluminum base and cover components. A leadframe is disposed between and bonded to both the base and cover. A silicon-based semiconductor integrated circuit is bonded either to the package base or to an intervening die paddle.
Electronic package designers have begun to replace the leadframe with circuitry deposited directly on a dielectric layer by processes such as direct writing, screen printing, electrolytic plating, electroless plating, vaporization, sputtering and vapor deposition. A ball grid array electronic package having circuitry deposited on an anodic film is disclosed in U.S. Pat. No. 5,629,835 by Mahulikar et al, that is incorporated by reference in its entirety herein. The circuitry is electrically isolated from a metallic aluminum alloy core by the anodic film. Defects in the anodic film may create an electrically conductive path from the metallic substrate to the surface of the anodic film. If an electrically conductive path develops, an electrical short circuit between the metallic core and the deposited circuitry is likely.
U.S. Pat. No. 5,534,356 to Mahulikar et al, that is incorporated by reference in its entirety herein, discloses that the break-down voltage of an anodized aluminum component may be increased by proper selection of the aluminum alloy core. The break-down voltage is the voltage that when applied across a dielectric layer, such as the anodic film, causes an irreversible degradation of that dielectric layer and is accompanied by an increase in electrical conductivity of several orders of magnitude. The patent discloses forming the core from aluminum alloys of the 1000, 5000 and 6000 series as designated by the Aluminum Association Incorporated.
The 1000 series alloys are 99%, by weight, or higher aluminum.
The 5000 series alloys contain magnesium and usually chromium as well.
The 6000 series alloys contain silicon and magnesium in approximate proportions to form magnesium silicide.
There remains, however, a need for aluminum alloy components for electronic packages that inhibit the formation of electrically conductive paths through an anodic film that may result in electrical short circuits between circuitry deposited on a surface of the anodic film and a metallic aluminum core.